The invention relates to a process for manufacturing, shaping and machining a shape memory alloy part, and the part obtained that has remarkable functional properties.
The technical field of the invention may be defined in general as being metallurgy and science of materials. More particularly, the invention is in the field of shape memory alloys, and their fabrication and shaping or machining.
Remember that a metallic alloy has a shape memory if, after permanent deformation when cold or during cooling under stress, it returns to its initial shape simply by heating. This phenomenon, called a single directional memory effect (EMSS) is caused by a martensitic transformation that occurs above a critical temperature called the xe2x80x9ctransition temperaturexe2x80x9d. This transition temperature may be adjusted between xe2x88x92200xc2x0 C. and +170xc2x0 C., by acting on the chemical composition and/or heat treatments.
Furthermore, martensitic transformation confers other particular properties on shape memory alloys (AMF) for example the capacity to generate a large force during heating; the super elastic effect, the rubbery effect, the assisted two-directional memory effect (EMDSA) and the two-directional memory effect (EMDS). The presence of the martensitic phase also significantly increases the damping capacities.
The performances of an AMF element can usually be estimated in terms of displacement amplitude and recovery force available when heating.
All specific properties of shape memory alloys explain the xe2x80x9cfunctional materialsxe2x80x9d or xe2x80x9cintelligent materialsxe2x80x9d terminology often used to qualify them.
The shape memory effect has been known in metallic alloys since 1930 and the first industrial application was in 1967, but due to the complexity of their behavior, their extreme sensitivity to fabrication conditions and their cost, their industrial and commercial development has remained very limited and mainly applies to sectors using state-of-the-art technologies such as the defense, space and medical equipment industries.
Fabrication procedures used to fabricate shape memory alloys depend on the required application and the final geometry required for the AMF part or element.
Shape memory alloy parts used for the main applications found so far have a simple geometry. There are usually bars, for example used in deployment mechanism actuators in space and actuators for robotics; for example, wires in dental fixtures, medical instrumentation, portable telephone antennas, spectacle frames, clothes; flats or strips in spectacle manufacturing, electrical activators in household automation equipment; or springs or xe2x80x9cstentsxe2x80x9d for stenosis in the medical field and xe2x80x9ctelltalesxe2x80x9d for detecting a failure in the cooling system in the food processing industry, etc.
The shaping processes used to obtain parts with the geometries mentioned above correspond to the use of conventional transformation means such as extrusion presses for producing bars; drawing and wire drawing benches to produce wires; rolling mills to obtain sheet metal, and shaping dies to make springs.
Machining processes may also be used for the purposes of any shaping operation by the removal of material intended to confer dimensions and a surface condition on an AMF part, for example a difference in shape and roughness within a given tolerance range.
In general, machining of the AMF takes place within the sequence of shaping operations, mainly at two levels:
either as a blank cutting operation for a blank to be rolled, forged, stamped or machined;
or as a finishing operation for previously extruded, drawn, forged parts, and parts assembled by welding.
Among the shaping processes mentioned above, processes that make use of cold shaping are difficult to implement on shape memory alloys, particularly on titanium-nickel alloys, and particularly due to the hardness of these materials.
In general, cold deformation must be limited to avoid damage and must include annealing steps at a controlled temperature to restore the material.
However, cold deformation is essential for these materials, because it provides not only precision on the final dimensional characteristics, but it also confers work hardening on the material that improves the mechanical properties and enables optimization of functional properties such as shape memory effect, the recovery force and super elasticity. It has also been demonstrated that as the work hardening effect increases, in other words as the deformation rate increases, the functional properties, and particularly the recovery force and the amplitude of the memory effect improve. Furthermore, after the last cold deformation phase, a short-term xe2x80x9cflashxe2x80x9d heat treatment must be carried out within a defined temperature range to optimize the functional properties.
The above confirms that the fabrication of finished parts, in other words parts with a defined geometry made of a shape memory alloy, requires long, complex and therefore expensive fabrication and shaping procedures, including cold transformation operations that are difficult to control.
Therefore, there is a need for a simple, fast and economic process for fabrication and shaping of a shape memory alloy part, comprising a limited number of steps and capable of producing parts with mechanical and functional properties at least equivalent to what can be obtained with known, long, complex and expensive processes.
The purpose of this invention is to divulge a process for fabrication and shaping of a shape memory alloy part that satisfies all the needs mentioned above, that does not have the disadvantages, defects, limitations and disadvantages of processes according to prior art and that solves problems that arise with processes according to prior art.
According to the invention, this and other purposes are achieved by a fabrication, shaping and machining process for a shape memory alloy part comprising a single step during which the fabrication, shaping and machining of the said part are done simultaneously in a single operation by a cutting machining process.
The process according to the invention satisfies all the needs mentioned above, does not have any of the defects of processes according to prior art and solves problems that arise with processes according to prior art.
Surprisingly, the inventors found that by applying a specific machining process, namely a cutting machining process, to shape memory alloys it is possible to perform fabrication, machining and shaping operations in a single step. Cutting machining processes are the only known machining processes capable of achieving the purposes mentioned above and solving problems according to prior art.
Cutting machining processes have the special feature that they preferably produce chips, unlike processes that remove material by abrasion, or by chemical or electrochemical methods.
Surprisingly, the cutting machining process used in the invention confers very high work hardening on the part or chip made of shape memory alloy and is simultaneously accompanied by extremely fast temperature rise that performs the same function as the heat treatment, this xe2x80x9cflashxe2x80x9d heat treatment actually corresponding to the final heat treatment performed in processes according to prior art. Consequently, with the process according to the invention, it is possible to obtain remarkable mechanical and functional properties on the chip in a single operation. The inventors have demonstrated that the properties of parts obtained by the process according to the invention, for example springs, are better than the properties that can be obtained on an AMF part or element obtained by a long and complex fabrication process according to prior art.
The process according to the invention is a means of directly obtaining the required remarkable properties after a single step.
Unlike processes according to prior art that involve a long series of complex shaping and machining steps, in most cases with a final cold deformation step followed by another heat treatment, the process according to the invention includes only one step during which all operations necessary for fabrication and shaping and machining of the part are done simultaneously. Furthermore, this single step is extremely simple, since it makes use of a simple, reliable and tested cutting machining process.
The process is extremely fast compared with processes according to prior art, since apart from the time saving obtained by minimizing the number of steps, the duration of the single step involving cutting machining is also reduced.
Consequently, the simplicity and speed at which the part such as a chip of shape memory alloy is made, will cause a very low unit cost compared with fabrication processes according to prior art. Consequently, broader xe2x80x9cgeneral publicxe2x80x9d applications, in the past impossible for shape memory alloys due to their prohibitive cost, could be considered for parts manufactured by the process according to the invention.
According to the invention, the cutting machining process is usually selected among high speed cutting machining processes, usually greater than 15 m/min and low speed cutting machining processes, usually at less than or equal to 15 m/min.
For example, high-speed cutting machining processes include milling, turning, traverse grinding, screw cutting, drilling, and low speed cutting machining processes include planing, tapping, forming mortises and broaching.
According to the invention, one or several machining parameters, in other words operational parameters of the cutting machining process, are varied in order to, make a part such as a uniform and continuous chip with a defined geometry and/or with required mechanical and/or functional properties.
This or these machining parameter(s) are usually chosen, for example, among the nature of the cutting tool, the pass penetration depth (or the cut thickness), the machine advance speed or the cutting speed, the angular position of the cutting tool, the sharpening of the cutting tool, and possibly the rotation speed of the blank from which the part is made.
Note that these parameters are usually not independent and are related to the geometry of the blank part used.
For example, the rotation speed depends on the geometry of the blank, and particularly the diameter of the blank, and depends on the capacities of the machine used. This speed may vary within wide limits. For example, a typical speed is 250 rpm.
The shape memory alloy used in the process according to the invention may be any known shape memory alloy. For example, it may be chosen among copper based alloys, titanium and nickel based alloys, and iron based alloys.
Copper based alloys include Cuxe2x80x94Zn, Cuxe2x80x94Al, Cuxe2x80x94Sn, Cuxe2x80x94Zn,xe2x80x94Al, Cuxe2x80x94Alxe2x80x94Ni, Cuxe2x80x94Alxe2x80x94Mn, Cuxe2x80x94Alxe2x80x94Be alloys. Tixe2x80x94Ni based alloys include alloys of titanium and nickel in all proportions, and preferably 50xe2x80x9450 Tixe2x80x94Ni alloys. The Tixe2x80x94Ni alloy may slightly be alloyed with one or more Fe, Cu, Zr and Hf additive elements.
Fe based alloys include the Fe3Pt compound, and Fexe2x80x94Nixe2x80x94Coxe2x80x94Ti and Fexe2x80x94Mnxe2x80x94Si alloys with additions of Ni and/or Cr if necessary. The term xe2x80x9cshape memoryxe2x80x9d alloy covers all alloys that do or could have this type of property, and xe2x80x9cshape memoryxe2x80x9d properties usually include single directional memory effect (EMSS) properties, two-directional memory effect (EMDS) properties, assisted two-directional memory effect (EMDSA) or super elasticity properties.
The invention also relates to a shape memory alloy part that can be manufactured by the process according to the invention.
More precisely, the said part is usually a chip, preferably uniform and continuous. Preferably, the geometry of this chip is defined, and the chip may be in the shape of a strip or ribbon, preferably with a precisely defined length, width and thickness, but it may also be in the shape of a wire, flat or others.
Furthermore according to the invention, the chip, preferably in the shape of a ribbon, may be wound directly at the end of the process without any additional step, in other words a spring is obtained without any additional operation, for example a helical spring preferably with adjacent turns can be obtained directly in a single operation, useable in all types of devices requiring shape memory alloy springs. This type of helical spring has excellent mechanical and functional properties, better than the properties of springs obtained by processes according to prior art.
The chip is also defined by its mass and volume and the spring, preferably with adjacent turns, is also defined by the diameter of the turns, the total number of turns and the number of turns used.
For example, the mass of the chip will usually be from 1 mg to 50 mg, the width of the chip in the shape of a ribbon will usually be from 1 or a few hundredths of mm (for example 2 to 9 hundredths) to 5 mm, and the thickness of the chip will usually be from 1 or a hundredths of an mm (for example 2 to 9 hundredths), to 1 or several tenths of an mm (from example 2 to 9 tenths).
In the preferred case, the chip is wound and therefore is therefore in the shape of a spring, preferably a helical spring and preferably with adjacent turns, the total number of useful turns is usually from 2 turns to 1 or a few hundredths of turns (for example 200 to 1000) and the total number of turns is usually from 2 to 1 or a few hundredths of turns (for example 200 to 1000), the diameter of the turns is usually from 1 to 15 mm.
As mentioned above, the part, in other words the chip, obtained by the process according to the invention, has remarkable mechanical and functional properties. One of the most representative functional properties is the efficiency that defines the work available per unit volume of the AMF.
The maximum efficiency of the part obtained by this process according to the invention, such as a helical spring with adjacent turns, is usually greater than 10 MJ/m3, and preferably from 10 to 15 MJ/m3, which is significantly better than the efficiency of parts such as helical springs prepared by processes according to prior art and clearly differentiates parts according to the invention from parts prepared according to prior art.
The invention also relates to any device or system comprising the AMF part or chip according to the invention. In particular, the invention also deals with an actuator or actuator/sensor type system or device comprising the AMF part according to the invention.
The shape memory alloy part or device according to the invention has single directional memory effect (EMSS), two-directional memory effect (ESDS), assisted two-directional memory effect (EMDSA) or super elasticity properties. The part prepared by the process according to the invention is thus used in many industrial sectors. In priority, it is aimed at domains in which the use of AMFs have not been widespread in the past, or were even unusable due to economic reasons alone, but the invention is also used in all industrial domains in which AMFs are already used conventionally.
For example, it could be used for many general public applications such as toys, decoration, sculpture.
Potential applications also apply to the miniaturization sector in which an AMF chip can be competitive with a mini-spring.
For example, this needs to be taken into consideration in micro robotics and in the automobile industry for which thermal activators are used in many applications such as radiator plug, fan disconnection, air conditioning regulator, brake cooling, etc.